Esterification of glycol ethers with carboxylic acids to prepare glycol ether esters is well known. The equilibrium reaction is typically performed in the presence of an acid catalyst. Water generated in the reaction is usually removed as a low-boiling component as the reaction mixture is distilled. Removal of water helps to drive the esterification toward completion. Azeotroping agents are commonly used to assist in water removal. Relatively nonvolatile glycol ether esters are often recovered by fractionation from unreacted carboxylic acid and glycol ether components, which are usually more volatile. The latter are then recycled to the reactor.
In a conventional glycol ether ester synthesis, the glycol ether and carboxylic acid, usually acetic acid, are fed to a reactor and partially converted (30-50%) to glycol ether ester and water. The reactor effluent is fed to a distillation column, and the products are separated. Water is removed overhead with the aid of an azeotroping agent. Without the azeotroping agent, significant amounts of acetic acid are removed overhead with the aqueous phase. Water and azeotroping agent are separated using an overhead phase separator, and the organic phase is returned to the column. Unreacted glycol ether and acetic acid are removed as a side stream from the column, and the glycol ether acetate product is recovered from the bottom of the column.
Unfortunately, acid-catalyzed esterification with many glycol ethers is complicated by numerous side reactions, such as ether hydrolysis and cyclization. The by-products formed in these side reactions complicate purification. The boiling points of these undesired materials are often close to the boiling points of the glycol ether and acetic acid. Conventional schemes designed to recycle unreacted acetic acid and glycol ether will also recycle these by-products. The concentration of by-products in the reactor is initially low, but because the by-products are recycled, their concentration in the reactor increases as a function of time, and eventually forces a shutdown of the unit.
An obvious solution to the problem is to fractionally distill the side stream containing glycol ether, acetic acid, and by-products. Unfortunately, some of the by-products often boil at a temperature between the boiling point of acetic acid and the glycol ether, so two additional distillation columns are required, adding cost and inconvenience.
It is therefore an object of the invention to find a way to remove by-products from the esterification reactor effluent. Another object of the invention is to remove the by-products in the overhead stream without overhead loss of glycol ether or carboxylic acid.